1. Field of the Invention
The present invention relates generally to a zoom lens and, more particularly, to a small-sized standard zoom lens constructed of a small number of lenses suitable for a standard installation into a 35 mm camera and having a large variable power ratio.
2. Related Background Art
In an interchangeable lens for a 35 mm still camera in recent years, there exists a zoom lens having a variable power ratio on the order of 1.8 including a standard field angle corresponding to an intermediate focal length of 50 mm among zoom lenses. Those zoom lenses are each constructed of two negative/positive lens groups having, sequentially from an object, negative refracting power and positive refracting power. It is felt that such a zoom lens has been completely established as a standard-installation lens in place of the standard lens (having a focal length of 50 mm or thereabouts if set in a 35 mm format). Consequently, this type of standard zoom lens is carried while being installed into the camera body as a commonly used lens. The indispensable condition is that the standard zoom lens should be reduced both in size and in weight at the minimum and should be low of price with a more sufficient image-forming performance.
The above-mentioned 2-group zoom lens is of the most suitable lens type for attaining the so-called standard zoom lens described above. In this connection, a variety of proposals have hitherto been given. Among them, U.S. Pat. No. 4,195,912 and Japanese Patent Publication No. 61-42246 and U.S. Pat. No. 4,560,253 disclose simple lens systems wherein a first lens group closest to an object is constructed of only two pieces of negative and positive lenses, thereby reducing the costs.
However, the zoom lens disclosed in U.S. Pat. No. 4,195,912 has a field angle of 52.degree. at the wide-angle end and a variable power ratio on the order of 1.7. The zoom lens disclosed in Japanese Patent Publication No. 61-42246 has a field angle of 54.degree. or 58.degree. at the wide-angle end and a variable power ratio on the order of 1.7 or 1.8. The zoom lens disclosed in U.S. Pat. No. 4,560,253 has a focal length of 36 mm at the wide-angle end and a variable power on the order of 1.9. Any zoom lens is insufficient in terms of both of the field angle and the variable power ratio as a standard installation zoom lens.
A current dominant version of the lens for a standard installation into the camera body is a standard zoom lens exhibiting a variable power ratio on the order of 2.1 or more including a standard field angle. The indispensable conditions are the miniaturization and the reduction in costs as in the above-described standard zoom lens having the variable power ratio of approximately 1.8. However, a zoom hens consisting of two lens groups has been so far optimal to a fulfillment of reductions both in price and in size while keeping a sufficient performance in the zoom lenses each having the variable power ratio on the order of 1.8. In the case of ensuring a variable power ratio on the order of 2 or greater, however, it is difficult to directly employ the small-sized zoom lens type consisting of the two lens groups as in the prior arts. Rises both in size and in the number of lens elements are invariably induced for keeping the image-forming performance well.
Besides, according to Japanese Patent Application No.3-251188 filed by the same applicant as that of the present invention, there is proposed a small-sized 2-group zoom lens constructed of an extremely small number of lenses and exhibiting a variable power ratio of about 2.2 and a good image-forming performance as well by overcoming the defects inherent in the conventional 2-group zoom lens. In the so-called standard zoom lenses in recent years, the main trend is on the verge of shifting to a zoom lens having a variable power ratio of approximately 2.7 including a field angle of about 74.degree.. It is very hard to attain this condition by directly using the conventional zoom lens type. A tendency is that the zoom lens is constructed of more lens groups and increases both in size and in costs.
Namely, when considered on the line of extension of the conventional technology, with the increased variable power ratio and field angle, the first lens group of the 2-group zoom lens is commonly constructed of at least three pieces of negative/positive lenses. Even when an aspherical surface is employed for compensating a distortion, it is difficult to compose the first lens group by two pieces of negative/positive lenses in the zoom lens having a wide field angle and a high variable power ratio. In addition, the lenses of the first lens group tend to increase in configuration. This largely contributes to an increase in size of the whole lens system. Besides, the number of lenses having a large diameter also increases. It is obvious that an adverse influence is to be exerted on a cost performance.
It is also required that the number of lens elements be reduced in terms of keeping veiling glare well by decreasing ghost and flare.
From the reasons elucidated above, the first lens group is constructed of two pieces of negative/positive lenses by decreasing the number of lens elements to obtain the compact standard zoom lens having a field angle of approximately of 74.degree. at a variable power ratio of about 2.7 and exhibiting a good image-forming performance at low costs. It is further required that an adequate refracting power arrangement and an aspherical surface be incorporated for a higher performance.
Besides, U.S. Pat. No. 4,195,912 and Japanese Patent Publication No. 61-42246 stated earlier disclose the zoom lenses wherein the first lens group is constructed of two pieces of negative/positive lenses. In those zoom lenses, though the variable power ratio is as small as 1.6.about.1.8, a total length and a diameter of the front lens element are both large. Especially at the wide-angle end, the total length becomes remarkably large. For this reason, an oblique ray incident on the front lens element passes through a location far away from the optical axis, and hence the diameter of the front lens element increases outstandingly. This results in such a defect that the lens system increases in size on the whole. Besides, the refracting power of the second lens groups is also weak. Therefore, the total length further elongates. A moving quantity of each group is also large. A fluctuation in comatic aberration and a curvature of field are not desirable in terms of aberration for largeness in size of the lens as a whole.
Still further, the zoom lens disclosed in U.S. Pat. No. 4,560,253 presents the following defect. The lens system has a relatively small total length and is therefore compact. However, the total length is large particularly at the wide-angle end. The oblique ray incident on the front lens element travels through the location far away from the optical axis, resulting in a dilation of diameter of the front lens element. This conduces to the increase in side of the first lens group as a whole, which in turn undesirably leads to a rise in the costs. Especially, the first lens group involves the use of the aspherical lens and therefore leads to a further rise in the costs.
As described above, the zoom lenses that have hitherto been proposed are each remarkably elongated in terms of total length at the wide-angle end. Those zoom lenses are, it can not be said, enough with respect to the compactness. Besides, the variable power ratios thereof are relatively small.